Method and apparatus for drying a particulate material such as bark

ABSTRACT

In a method and apparatus for drying a particulate material, for instance bark, the material is disposed as an annular bed (11) on a perforated disc (10). The bed is moved along the disc by being rotated about its axis by pushing members (12). A surface layer of moist material is continuously deposited on the upper surface of the bed in an area extending over the entire width of the bed, while at the same time a corresponding surface layer of dried material is continuously removed from the lower surface of the bed in a substantially corresponding area (17), so that the bed thickness is substantially maintained uniform over the entire area of the bed. A uniform moisture content is imparted to a drying medium which is passed through the support disc and the bed, the medium is preferably saturated with moisture before leaving the bed.

TECHNICAL FIELD

The invention relates to a method and an apparatus for drying aparticulate material such as bark, and particularly to a method andapparatus including a substantially horizontal support for carrying abed of such material, means for passing a drying medium upwards throughthe support and the bed, and means for feeding new material to anddischarging dried material from the bed.

BACKGROUND ART

Material is traditionally dried by a drying medium in the form of a gas,the supply temperature of which it is attempted to be kept high in orderto minimize the costs for the drying apparatus itself, i.e. chiefly thesize of the apparatus. In such a case the drying medium will depart fromthe apparatus at a relatively high temperature, and above all in amoisture-unsaturated state, which means that it is seldom possible toachieve the optimum energy utilisation.

In order to achieve the greatest possible contact area between materialand drying medium the material is often dried in a suspension of saidmaterial and said drying medium. This often results in large investmentsfor separating the drying medium and the dried material. Favourablemoisture content and temperature gradients between drying medium andmaterial can seldom be reached in suspension drying, due to anuncontrolled mixing of the material. A drying technique of the type justmentioned is described e.g. in the Swedish patent application Nos.7810558-2 and 8307170-4.

Another known method of drying bark comprises spreading out a layer ofmoist bark to form a bed on a gas permeable support and passing a dryingmedium upwards through said support and said bed, as well as linearlymoving the bed over the support through which drying medium is blown,new bark being supplied to the bed at its rear end and dried materialbeing removed at its front end. Problems will then occur, since the bedat its front "dried" end has a certain moisture gradient or profile,which means that the moisture content of the removed material is somekind of a mean profile value. Additionally, since the flow resistance ofthe bed often declines with dropping moisture content therein, thedrying medium rather will tend to flow through the relatively dry frontor forward part of the bed, the result being that the drying process"gallops" at the front end part of the bed. In turn, this results in anumber of drawbacks, e.g. a low moisture content in a large proportionof the drying medium departing from the bed, bringing the requirementfor large apparatuses and large energy consumptions for heating and/ordehumidifying the drying medium, if it is to be recirculated through thebed. Furthermore, it is difficult to regulate drying such that removedbed material has the optimum moistness. The temperature and moisturegradients between drying medium and material will also not be theoptimum one.

As other prior art which shows other feeding and removal techniques perse reference can be made to FI 66485 which describes an apparatus thatworks with an endless wire to discharge the material to be dried. Fromthe figure it can be gathered that a thin layer is removed from thebottom of the bed by the means 13 and 14 which are operated in anoncontinuous manner. To operate such a process continuously it shouldmost probably be necessary to remove layers that are very thin whichmight be possible for very homogeneous materials like cereals or similarbut not for bark or similar materials.

Accordingly, one object of the invention is to provide a technique bywhich the material, arranged as a bed, is dried in conditions such thatthe drying medium departing from or leaving the bed has a substantiallyconstant or uniform moisture content and is preferably saturated withmoisture, while at the same time material with a desired degree ofdryness can be taken from the bed.

SUMMARY OF THE INVENTION

The method according to the invention of drying a particulate material,e.g. bark, is a method of the type where a bed of said material isarranged on a drying mediumpermeable support, a drying medium is drivenupwards through said support and said bed, and during the dryingoperation dried material is discharged from and new material is fed tosaid bed, and is characterized in that the bed is arranged as an annularbed on a stationary support, the bed is moved in its circumferentialdirection on the stationary support, dried material is removed from thelower surface of the bed through said support while the bed is movingand new material is deposited on the upper surface of the bed whileessentially maintaining a uniform bed thickness, and the drying mediumis passed through the bed in conditions such that when leaving the bedsaid drying medium has a substantially uniform moisture content inessentially all part areas of the bed, permeated by said drying medium.

The method is preferably carried out in such a way that the dryingmedium is in a substantially saturated state when departing from thebed.

The take-up of moisture of the drying medium originates substantiallysolely from the moisture of the bed material.

Preferably the drying medium is passed through the whole area of thebed.

By removing a layer of dried material from the lower surface of the bedthe advantage is achieved that the layer of the bed which is thereabovecan be moist, so that the drying medium can reach a uniform moisturecontent on departure from the bed, and can especially be moisturesaturated. This means that the flow of drying medium can be minimisedand that optimum moisture and temperature gradients between material andmedium are obtained. Since the drying medium departing from the bed hasa uniform moisture content, and is preferably saturated, the medium canbe effectively dehumidified by an optimum dehumidifying apparatus and bereheated for recirculation through the bed. A heat pump may then beutilised in a manner known per se for dehumidifying and reheating themedium. When recirculating the drying medium, whether or not it consistsof steam or any gas, heat exchanger(s) (heater(s)) can be utilised inthe recirculation duct, the heat exchanger(s) heating the gas to thedesired temperature or superheating part of the steam flow departingfrom the bed (any excess of saturated steam after the bed is dischargedas a practically dry, saturated steam).

When depositing material on and removing material from the bed the meansused therefore is displaced or moved relative to the bed. Saiddepositing and removal means extend over the entire width dimension ofthe bed, so that material is deposited or removed, respectively, overthe width of the entire bed in the longitudinal displacement of the bedrelative to said means.

During operation the bed has a moisture profile that varies along thedisplacement direction of the bed. Generally the bed has a uniformthickness over its entire area. The drying medium through-flowresistance of the bed depends on the moisture profile of the bed, thehigher the moisture content in the bed the greater the flow resistance.In addition thereto, a thick moist bed layer can saturate a greaterdrying medium flow with moisture than a relatively thin, moist bedlayer. In accordance with the invention it can therefore beadvantageous, particularly with greater drying medium flows through thebed, to arrange the drying medium flow proportionally greater in thepart areas of the bed where the bed has a relatively higher moisturecontent. In this way the bed volume of the apparatus can be utilisedoptimally with respect to the achievement of a uniform moisture contentin the drying medium, which departs from all the part areas of the mainsurface of the bed. Since the gas flow resistance of the bed isdependent on the thickness thereof, the bed thickness should be keptuniform. This can be achieved by having material removed and depositedat substantially a single position, although at opposite main surfacesof the bed. As mentioned the bed is annular and is displaced in itscircumferential direction on a fixed, horizontal support or carrier,e.g. a perforated disc, by means of driving or forwarding means.Deposition of material can be accomplished using a radially extending,stationary screw feeder which, apart from supplying new material oversubstantially the entire width of the bed, also "ploughs off" the uppersurface of the bed, thus maintaining a uniform bed thickness. The screwfeeder is preferably arranged to maintain a material through-flow.Material discharge or removal can be obtained by using an opening or aslit extending across the width of the support to define the inlet of adischarge chute, in which a discharging screw conveyor can be disposed.To ensure that a uniformly thick bottom layer is removed from theannular bed, the screw of the discharge conveyor may have a pitch thatcan be varied to suit the different material flows to be taken out atdifferent bed radii. Alternatively, for different radial bed sectionsthere may be different discharge chutes with associated screw conveyorsof different capacities for achieving the situation that a uniformlythick bed layer is discharged over the entire bottom side of the annularbed. The bas-permeable support is preferably horizontal.

An apparatus for carrying out the method claimed includes a gaspermeablesupport on which a uniformly thick particulate material bed issupported, means for displacing the bed along the support, means forpassing a drying medium up through said support and said bed, means forremoving a uniformly dried layer from the bottom side of the bed, andmeans for depositing new material on the upper surface of the bed so asto deposit on the bed an amount of material which corresponds to theamount of dried material removed, while maintaining a substantiallyuniform layer thickness.

Preferably, the apparatus also includes a housing, which defines acirculation flow path for the drying medium. In said flow path there aresuitably fans for driving the medium through the bed, and heating meansfor reheating the medium as well as dehumidification means fordehumidifying the drying medium before the recirculation thereof.Reheating and dehumidifying the drying medium can be achieved in anyconventional manner by a heat pump, the vaporiser of which dehumidifiesthe drying medium by cooling the same so that the condensate can beremoved, while the thus dehumidified gas is reheated at the heat pumpcondenser. Alternatively, the drying medium can be passed through asocalled ADIAC absorber, in which the drying gas is simultaneouslydehumidified and heated (see Swedish Pat. No. 7902979-9).

The invention, as well as preferable embodiments thereof, are defined inthe accompanying claims.

An embodiment of the invention will now be described in more detail withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a vertical section through dryingapparatus in accordance with the invention.

FIG. 2 is a section along the line II--II in FIG. 1.

FIG. 3 illustrates an arrangement for varying the air flow throughdifferent sections of the main surface of the bed in its direction ofmovement.

FIG. 4 is a schematic section taken along the lines IV--IV in FIG. 1 andillustrates means for supplying material to and discharging materialfrom the bed.

EMBODIMENT SHOWN IN DRAWINGS

In FIG. 1 there is shown a generally circular-cylindrical apparatus inaccordance with the invention and intended for the drying of bark. Theapparatus includes a generally annular housing the outer diameter ofwhich may approach about 20 meters. In the housing there is a gaspermeable support in the form of an annular, horizontal, perforatedplate or disc 10, which carries a uniformly thick bed 11 of bark. Saidperforated disc 10 is stationary, and the bed 11 is brought to arotational movement by the pusher paddles 12 depicted in FIG. 2. Thesepusher means 12 are driven by drive means 13 to give a rotational speedof for instance one revolution per three hours for bark. Below theperforated disc 10 in the housing there is a distribution chamber 15 forthe drying medium. Furthermore, above the bed 11 there is in the housinga collection chamber 20 for drying medium that has passed through thebed 11. There is also a schematically illustrated recirculation duct 30that brings the chambers 15,20 into mutual communication with eachother. A unit 31 is arranged inside the duct 30 for dehumidifying andheating the drying medium. Additionally there is a fan 32 forrecirculating the drying medium.

The thickness of the bed 11 may be between 200 and 2000 mm. Since thedrying medium is passed upwards through the bed 11, the friction betweenthe bed 11 and the support 10 will be low. Material depositing means 50in the form of a screw feeder 51 is illustrated in FIG. 4, which screwextends radially from an external radially situated supply hopper 52 andopens at the central, vertical through space 7 in the drying apparatus.With the aid of a schematically illustrated conveyor 55 bark material isrecirculated to the hopper 52; the material is conveyed in excess by thescrew 51 through the apparatus housing, inter alia to maintain in asimple way a uniform bed thickness therein.

Dried material is removed from the bottom side of the bed via one ormore generally radially extending removal slits 17 in the support 10,which slits 17 form input openings to a chute 18, from which fallingdried material is removed by discharging means such as screws 19. Threescrews of different capacities are illustrated in FIG. 2 for the removalof a material layer of the same thickness over the width of the entirebed, but it should be clear that there could be only one radiallyextending slit 17 for a corresponding chute 18, in which there is ascrew conveyor, the pitch of which varies along the length thereof sothat the discharged layer will have a uniform thickness across the widthof the bed. As will be seen from FIG. 4, the discharge means 17.18.19and the material depositing means 50 should be situated directlyopposite each other at either side of the bed so as to substantiallymaintain the thickness of the bed.

The distribution chamber 15 is schematically illustrated in FIG. 3 asbeing divided by partitions 60 into a plurality of chamber sectors orsegments 15' separated from each other in the circumferential directionof the housing. One partition 60 is situated in the same circumferentialposition as the material depositing means and the material removalmeans. Each of the chamber segments 15' is assigned its own fan 32' forthe drying medium. The different fans 32' are adjustable to givedifferent flows. Briefly while referring to FIG. 4 it can be seen thatin the rotational direction of the pusher means 12 the bed hassubstantially greater moistness immediately after the depositing means50 than immediately before the same. By dividing the distributionchamber 15 in a number of sectors, which are separated in thecircumferential direction of the bed, and assigning each such sector itsown drying medium fan 32', it can be ensured that even for great dryingmedium flows the medium departing from the bed has a uniform moisturecontent in all part areas of the bed, by corresponding flow adjustmentsof the fans 32' assigned to the respective chamber sectors 15'. By meansof the individually controllable fans 32' a drying medium throughflowcan be provided in the different circumferential sections of the bedsuch that the medium will obtain a predetermined moisture content ondeparture from the bed in all the part areas of the bed through whichgas flows, these being simultaneously achieved as compensation forvarying drying medium through-flow resistances in the circumferentialdirection of the bed, which depend on the moisture content of the bed.

What is claimed is:
 1. A method of drying a particulate material,especially bark, comprising:arranging an annular bed of said material onan annular drying medium-permeable stationary support; passing a dryingmedium upwards through said support and said bed; moving said bed in thecircumferential direction thereof on said stationary support; removingdried material from the lower surface of the bed through said supportwhile the bed is moving and depositing new material on the upper surfaceof the bed while essentially maintaining a uniform bed thickness; andpassing said drying medium through said bed such that when leaving saidbed said drying medium has a substantially uniform moisture content inessentially all part areas of said bed permeated by said drying medium.2. The method as claimed in claim 1, wherein said drying medium isbrought to an essentially moisture-saturated condition before leavingsaid bed.
 3. The method as claimed in claim 1, additionally comprisingthe step of varying the drying medium-pressure difference over differentpart areas of said bed to compensate for varying moisture-contentdependent through-flow resistance of said bed.
 4. The method as claimedin claim 1, additionally comprising the steps of displacing a materialremoval means and a material depositing means relative to said bed;andoperating said removal and depositing means to remove and depositmaterial, respectively, in similar flows at corresponding positions soas to essentially maintain the thickness of said bed.
 5. The method asclaimed in claim 4, wherein said removal means is provided with at leastone inlet gap stationarily disposed in said support.
 6. The method asclaimed in claim 1, wherein said drying medium is passed throughsubstantially the entire area of said support and said bed; andsaiddrying medium is preferably recirculated through said bed after adehumidification and heating operation.
 7. The method as claimed inclaim 1, wherein said drying medium is a gas which is dehumidified andreheated before being recirculated.
 8. The method as claimed in claim 7,wherein said gas is dehumidified and reheated by means of a heat pump.9. The method as claimed in claim 1, wherein said drying medium is steamwhich is passed into said bed in a superheated state.
 10. An apparatusfor drying a particulate material, especially bark, comprising:asubstantially horizontal support for carrying a bed of said material,said support and said bed having a substantial annular configuration;means movable over said support for moving said bed in a circulardirection on said support; means for passing a drying medium upwardsthrough said support and said bed and for bringing said drying medium toa substantially uniform moisture content when leaving said bed in allpart areas of said bed which are permeated by said drying medium; meansfor discharging dried material from said bed, said means comprising atleast one inlet opening in said support, which opening extendstransversely to the direction of movement of said bed for removing auniformly thick, dried material layer from the lower surface of said bedwhen said bed is moving in circular direction; and means for feeding newmaterial to said bed and for depositing said new material on top of saidbed to compensate for said removal of said dried material so as toessentially maintain the bed thickness.
 11. The apparatus as claimed inclaim 10, additionally comprising a drying medium distribution chamberbelow said support, said chamber being divided into a number of chambersections which are separated from each other in the direction of themovement of said removal means relative to said bed; and an individuallycontrolled drying medium fan at each said chamber section.